This invention relates to the tinning of the leads of an IC package with solder from a solder wave to prepare them for soldering to a printed circuit board or the like. More particularly, the invention relates to an improved solder wave apparatus that eliminates the undesirable "solder splash," characteristic of solder waves, which "solder splash" can cause electrical shorts between the traces or leads of an IC package.
An integrated circuit chip is typically a rectangle of processed silicon with dimensions on the order of a tenth of an inch. In order to test and use the chip, it is placed in an IC package, wires are bonded between the input/output pads of the chip and the leads of the IC package, and the chip is sealed within the package in some manner.
The packaged IC is then tested and affixed to the next level of packaging, typically a printed circuit board, for its intended use. If the number of leads on the IC package is small, it is usually affixed to the printed circuit board by inserting the leads of the package through plated holes provided on the printed circuit board and soldering the leads to the plated holes.
As the number of leads on an IC package is increased, the leads, by necessity, become smaller and more easily damaged by handling. IC packages with a large number of leads are typically affixed to the printed circuit board by lap soldering the leads of the package to pads on the board. Advantageously, the manufacturing process associated with lap soldering techniques allows each lead to be aligned with the pad to which it is to be soldered, without damaging the lead, before the lap soldering process takes place. In the case of removal of the packaged IC, special tooling is used to unsolder all of the leads at once so that the package can be removed without damaging the leads of the package or the pads on the printed circuit board.
IC packages which are lap soldered are typically fabricated with their leads gold plated. This is done because a solder plate on the leads has a definite "shelf life", i.e., solder plate will oxidize over a period of time and, if oxidized, the leads of the package may not lap solder properly because of the oxidation. Also, it is difficult to plate enough solder on the leads to provide the amount of solder necessary for the lap soldering process.
At some point in the manufacturing process, prior to the lap soldering process, the gold plated leads of the IC package are "tinned". This is usually done by momentarily dipping the leads in a solder wave of a solder wave machine. The molten solder from the solder wave adheres to the gold plated leads and provides enough solder for each lead to be lap soldered to the pads on the printed circuit board.
An undesirable characteristic of a solder wave is that as the surface of the wave is exposed to the atmosphere, globules of molten solder leap out of the wave. This phenomenon is called "solder splash". The solder splash is flung into the air only an inch or so, but the globules can strike the exposed gold traces of the IC package, above the leads being tinned, and cause electrical shorts between the traces. When this occurs, the electrical shorts must be removed by removing the adhering solder splash.
The existance of solder splash significantly adds to the manufacturing cost of the packaged IC. This is because, first, the electrical shorts must be found; and secondly, they must be removed. Various schemes have been devised to find such electrical shorts, such as continuity checking between the leads after they have been tinned; but in some cases, the number of electrical shorts, and the expense of trying to remove them, can be so great as to cause the package to be discarded rather than attempting to remove the shorts.
The present invention provides an improved apparatus that eliminates the solder splash from the solder wave so that the troublesome and expensive electrical shorts do not occur. The result is a significant reduction in the time and cost of the IC packaging process.